RDFT and DRM in DIGITAL SSTV explained

Redundant Digital File Transfer and Ham Digital Radio Mondiale


RDFT (Redundant Digital File Transfer is the 1st attempt for sending DIGITAL SSTV pictures oh HF.
Barry Sanderson, KB9VAK, has developed a multi-channel, multiphase modulation scheme known as Redundant Digital File Transfer (RDFT). Barry also wrote the core software routines that allow RDFT to run on personal computers using sound card DSP capabilities. This allows error-free< transmission of computer files via standard amateur radio equipment. RDFT has been proven very powerful in the area of DIGITAL Slow-Scan Television.


DRM (Digital Radio Mondiale) is a method of broadcasting on AM radio using digital rather than normal analogue signals. If you have ever listened to an audio stream on the Internet you will know waht it is. Of course DRM signals are broadcast on Mediumwave or Shortwave radio, not on Internet.

Except for potentially crystal-clear reception, DRM signals can carry multimedia information in addition to the audio signal, or to carry multiple program streams on one signal.

This method has been modified by HB9TLK who created HamDRM and today it is used mainly on the HF bands with some success by quite a few Radio Amateurs.

According to PY4ZBZ HamDRM carriers use OFDM (Orthogonal Frequency Division Multiplex) modulation. The modulated carriers are overlapping in frequency domain and are modulated in QAM (amplitude and phase). In the current state of RDFT, the transmission of a file needs three separate operations, Coding, Recording (of the transmission) and Decoding, and this increases the total time of transmission/reception.
In HamDRM, a file is codified and transmitted simultaneously, and it is decoded at the same time when it is received. There is only a small delay between the end of the transmission and the reception, (a few seconds).
To transmit a file needs no more separate coding and decoding as in RDFT. Just load the file (if not to big), or use Jp2 converter for adjust the picture file a suitable size in bytes.
The file is divided in blocks (RDFT) or segments (HamDRM). The size of segments depends on the mode particularities.

Mode A is faster than Mode B but it doesn't fight negative effects caused by multi-path propagation as Mode B, which is more robust.
QAM modulation can be in 4 or 16 or 64 vectors or symbols.
64 QAM is much faster but needs a very good signal to noise ratio (SNR), or a high quality channel.
4 QAM is the slowest but it supports more noise, or smaller SNR.
Interleaving consists of changing the natural symbols sequence.
If it is long (2 seconds), it allows better error correction with slow fading, but causes a little larger delay in the reception. Short interleaving is 400 ms.

The occupied bandwidth BW for the HamDRM signal can be set to 2,3 or 2,5 kHz. The smallest is a little slower and it should be used if the SSB TX/RX equipments filters are to narrow or have not a flat transfer function.
The DC frequency simply refers to the inferior limit of BW, and also should be chosen in agreement with the equipments filters.
The amount of instances is the number of times that a file is transmitted during one transmission. More than 1 instance allows that if during the reception of the first instance some segments were not received, they will probably be received during the second or third instance, correcting the received file.
Leadin is just a extra time of initial transmission so that the receiver can have time to synchronize correctly.
BSR is an adapted system from BBR of DIGTRX and allows to do a request to send only for the missing segments of a received file.

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